1. Field of the Invention
The present invention relates to fixing apparatuses that are applied in electrophotographic image forming apparatuses such as copiers, facsimile machines, and printers.
2. Description of the Related Art
Generally, in image forming apparatuses such as copiers, printers, and facsimile machines, an electrostatic latent image is formed on a photosensitive body, toner is supplied from a development apparatus to the photosensitive body, and a toner image is formed on the photosensitive body by the electrostatic latent image on the photosensitive body being developed by the toner. Thereafter, the toner image is transferred from the photosensitive body to a sheet of recording paper, and heat and pressure are applied to the sheet of recording paper to cause the toner image to be fixed to the sheet of recording paper.
In the fixing apparatus, the sheet of recording paper is sandwiched between a hot roller and a pressure roller (also referred to as fixing rollers) and subjected to heat and pressure, which causes the toner image to thermally fuse and become fixed onto the sheet of recording paper. To reliably fix the toner image onto the sheet of recording paper it is necessary to suitably regulate the surface temperature of the hot roller.
Incidentally, along with increased speeds in image forming apparatuses, increased speeds are also continuing to be achieved in fixing apparatuses. For example, in dual-use color and monochrome image forming apparatuses, there is a higher frequency usage of monochrome printing over color printing and since processing speed is given priority over image quality for monochrome printing, higher speeds of print processing are enabled more for monochrome print processing than color print processing, thereby increasing the usefulness for monochrome print processing. Specifically, when using standard A4 sheets of recording paper, the transport speed for the sheets of recording paper during color print processing is set to 41 sheets/minute (a processing speed of 225 mm/sec) and the transport speed for the sheets of recording paper during monochrome print processing is set to 70 sheets/minute (a processing speed of 350 mm/sec), thereby increasing the speed of monochrome print processing.
However, when switching the transport speed or processing speed for the sheet of recording paper in this manner, the surface temperature of the fixing rollers tends to drop for increases in the transport speed or processing speed for the sheet of recording paper, which makes deficiencies in fixing of the toner image onto the sheet of recording paper more likely to occur.
This is because a pass-through time of the sheet of recording paper passing through a nip region between the hot roller and the pressure roller becomes shorter for increases in the transport speed or processing speed for the sheet of recording paper, and the amount of heat necessary for fixing the toner image onto the sheet of recording paper cannot be transferred between the sheet of recording paper and the rollers such that deficiencies occur in the fusing of the toner onto the sheet of recording paper. This is also because when the transport speed or processing speed for the sheet of recording paper is increased, the distance of separation between the sheets of recording paper becomes shorter, such that the time in which a sheet of recording paper is present in the nip region between the hot roller and the pressure roller becomes longer, thereby shortening the time in which the hot roller and the pressure roller are in direct contact, such that the amount of heat conducted from the hot roller to the pressure roller is reduced, thereby lowering the surface temperature of the pressure roller, and the amount of heat conducted to the sheets of recording paper passing through the nip region is reduced, which causes deficiencies in the fusing of the toner onto the sheet of recording paper. As a result, reductions in print quality are incurred.
For this reason, in JP 2004-198659A for example, an endless heating belt is brought into contact with the fixing roller and set so as to widen the contact surface area therebetween, thus making it possible to supply a sufficient amount of heat from the heating belt to the fixing roller.
Also, in JP 2004-85601A, external hot rollers are provided to contact each of the fixing rollers respectively and the amount of heat produced by each of the external hot rollers is controlled to achieve control over the heating of the surface of each of the fixing rollers by the respective external hot rollers, thereby maintaining the surface temperature of each of the fixing rollers.
However, in cases where the surface temperature of the fixing rollers is controlled by bringing a heating belt or an external hot roller into contact with the fixing rollers as in the above-mentioned JP 2004-198659A and JP 2004-85601A, when the surface temperature of the fixing rollers drops along with increases in the transport speed or processing speed for the sheet of recording paper as described above, the temperature of the heat source of the heating belt or the external hot roller or the like must be raised quickly to suppress drops in the surface temperature of the fixing rollers, and this increases the load on the heat source itself and the control circuits for the heat source.
Furthermore, since the contact surface area between the heating belt or the external hot roller and the fixing roller is kept constant, the heat of the heating belt or external hot roller will not always be conducted quickly to the fixing rollers even when the temperature of the heating belt or external hot roller is raised, which makes it difficult to control the surface temperature of the fixing rollers. For this reason, it is conceivable to widen the contact surface area between the heating belt or the external hot roller and the fixing roller to improve the thermal conduction between these, but doing this involves permanently increasing the resistance against the rotation of the fixing rollers, which increases the load on the rotational mechanism or the like of the fixing rollers.